Actuaries define senescence as the increased likelihood of dying, but the appropriate pathological definition is progressive, multi-system organ atrophy. This is obviously not atrophy of disuse but atrophy of what can be called "cell drop-out". "Cell drop-out" as used herein means the disappearance of cells and consequent shrinkage of the body's organs. Such organ shrinkage is seen in autopsies on the aged. For example, the human brain can decrease from an average weight of 1,500 grams in a young human adult to a 1,000 grams or less in a human of advanced age. A brain that has decreased in size (the senescent brain) is highly forgetful, is unable to memorize new information and cannot react quickly to external stimulae. Shrinkage with age is also found in other human organs such as the heart, liver, kidneys, lymph nodes, skeletal muscles and vertebrae. Such shrinkages often correspond to the accumulation of peroxidized, free radical products seen as brown-yellow age pigment (lipofuscin). Other corresponding changes that are associated with aging are wrinkled skin, depleted fat deposits, fewer dermal melanocytes, brittle bones, low infection resistance, poor exercise tolerance and lack of reproductive ability.
At the cellular level, senescence means inadequate DNA repair leading to disordered and/or nonexistant cell replication, loss of mitotic control factors in the nucleous and cytoplasm including disordered nuclearcytoplasmic exchange, and permanent closing of microcirculatory capillary beds resulting in focal cell drop-out and loss of cell and organelle membrane function. This progressive cellular process affects all organs and tissues throughout the body and its etiology and pathogenesis must therefore involve a universal and fundamental aspect of cell physiology.
Aging of the mammalian organism as a whole must be examined at the cellular level because changes of individual cells affect changes in individual body organs and changes in individual body organs affect the organism as a whole. For example, death or dysfunction of a critical bodily organ, such as the heart, will result in the death of the body as a whole.
In any study on aging two distinct types of cells must be considered; normally dividing cells and postmitotic cells. Normally dividing cells are those of the skin, hair and gastrointestinal tract, for example. While thousands of such cells die daily, they are continually replaced. Their replacement is with nearly exact replicas until the time of aging or senescence begins. This time begins in the mid-twenties in man. The second cell type, i.e. post-mitotic cells, are those that make up the heart, brain and central nervous system, for example. Generally speaking, post-mitotic cells don't divide or reproduce. Mammals are born with a fixed number of post-mitotic cells which lose function and die daily throughout a mammal's life span. Death of a mammal occurs when a critical number of post-mitotic cells lose function or die in a critical organ, e.g. the brain.
Recent biomedical gerontological research has provided some theories regarding the metabolic course of events which leads to the ineviable loss of function, deterioration, destruction and death of mammalian cells. One convincing theory relates to mammalian cellular metabolism's reliance on oxygen metabolism. By "oxygen metabolism" is meant the burning of oxygen in the cells energy factories (mitochondria) together with foods such as fatty acids to produce adenosine triphosphate (ATP), the cell's energy source. Production of ATP occurs continually through enzymatically controlled chemical reactions. Unfortunately, these chemical reactions are not 100% efficient. For example, in the bacteria, E-coli, these reactions are only approximately 84% efficient. As one moves up the scale of evolution, these reactions become increasingly more efficient. In man the reactions are 94.+-.2% efficient.
More efficient "oxygen metabolism" in man as compared to other mammals results in a decrease in the burning rate of oxygen in man compared to other mammals, a decrease in the burning rate of fatty acids and carbohydrates, a decrease in the formation of toxic oxygen byproducts and a decrease in the temperature set-point of the body to below that of other mammals.
Even though the relative amount of byproducts produced by "oxygen metabolism" in man is less than the amount of such byproducts produced in other mammals, such byproducts are highly toxic. Examples of these toxic byproducts are the superoxide (O.sub.2.sup.-.) and hydroxyl (.OH) radicals. To illustrate the extreme toxicity of these radicals consider the following: When superoxide is mixed with water, concentrated hydrogen peroxide (an oxidative byproduct) is formed. Concentrated hydrogen peroxide poured onto the skin will damage it within a few minutes to a necrotic, sickly white appearance. The same chemical reactions occur inside mammalian, oxygen-burning cells. Although protective enzymes destroy hydrogen peroxide continually, such defenses are not 100% efficient. Thus, chemical destruction of cells can result. Since man consumes many pounds of oxygen daily, even a slight inefficiency in the use of oxygen resulting in the O.sub.2.sup.-. or .OH radicals and oxidative byproducts is significant. As used herein "toxic byproducts of oxygen metabolism" includes radicals such as superoxide (O.sub.2.sup.-.) and hydroxyl (.OH) as well as oxidative products produced by such radicals such as hydrogen peroxide and the like.
An example of cell damage caused by the byproducts from oxygen reactions, i.e. by toxic byproducts of oxygen metabolism, is damage to a cell's genetic material, DNA. It is known that approximately 7,000 DNA "hits" by toxic byproducts of oxygen metabolism occur daily in man. Fortunately, most of these are repaired enzymatically by the cells. It is known, however, that some cells that are not repaired may become cancerous.
Of the two previously described cell types, i.e. the normally dividing cells and post-mitotic cells, normally dividing cells are able to reproduce and replace damaged cells. Post-mitotic cells, on the other hand, while they are sometimes repaired after bombardment with toxic oxygen byproducts of oxygen metabolism, most often such bombardment reduces their function, damages their DNA and/or kills them outright. This is a steady-state, linear aging process until damage to DNA and cellular defensive enzymes becomes so great that the aging process acquires an endogenous, accelerating character.
The long term, lifetime effects of this endogenous toxic chemical bombardment can also be exemplified in the wrinkling and hardening of the skin and arteries with age. The skin and arteries consist of supportive material called collagen and elastin. Collagen is the major protein of the white fibers of connective tissue, cartilage and bone. Elastin, or elastic tissue, is the major connective tissue protein of elastic structures such as the large blood vessels and the skin. Elastin enables these structures to stretch, and then resume their orignal shape and size.
Collagen and elastin contain fibers internally linked together by chemical bonds called a "imide bonds". It is theorized that mammalian aging involves the oxidation of these imide bonds to "amide bonds". In the skin and arterial collagen and elastin of mammals, as more and more amide bonds are formed, the collagen and elastin fibers become increasingly less elastic and flexible. In man, it is known that these fibers harden at a rate of approximately 7% per decade after the age of maturity (approximately mid-twenties). This means that the arterial-vascular system has a theoretical life span of approximately 140 years before becoming 100% rigid.
Free radical pathology mechanisms seem to be involved at key points in the etiology and pathogenesis of cancer, occlusive atherosclerosis and wrinkling of the skin. Free radical pathology ensues largely from free radical and oxidation products which negatively affect cell membranes, collagen, elastin, immune functions, micro-circulation, nucleic acids and regulatory proteins. Scientific studies have shown that this mechanism progresses from inciting factors to free radical pathologic reactions to products to damage to organelles, cells and tissues resulting in aging and disease. Aging seems to simultaneously affect all cells, tissues and organs throughout the body in an insidious, progressive pattern whose pathogenesis and etiology therefore are thought to involve a fundamental and universal aspect of cell physiology.
The clinical significance of toxic free radicals and oxidative agents generated endogenously in living cells has been documented in various scientific, medical publications. An association between these toxic agents and aging has been observed.
It is therefore desired that methods be provided for inhibiting aging caused by free radicals and oxidative agents produced in living cells. Such methods desirably include systemic treatment of humans to inhibit aging of the entire body as well as topical treatment, e.g., to inhibit wrinkling/aging of the skin.